Importer API

There should be an easier way to import programmatically.

``` python
lib = beets.Library(path=settings.DB_PATH, directory=settings.MUSIC_PATH)
import_files(lib, [settings.MUSIC_PATH], None)
```

Importing this way requires user input.
The should be a way to do this programmatically without user input and without hunting through the source.

Start by tracing the shown import_files(lib, [settings.MUSIC_PATH], None) entry point and how it requests user input. Define the programmatic import behavior and verify that callers can import files without interactive prompts or searching through internal source code.

Written by the indexing model from the issue text.